#include <unistd.h>
#include <stdio.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <fcntl.h>

typedef struct{
	int integer;
	char string[24];
} RECORD;

#define NRECORDS 100

int main()
{
	RECORD record, *mapped;
	int i, f;
	FILE *fp;
	
	fp = fopen("records.dat", "w+");
	for (i = 0; i < NRECORDS; i++)
	{
		record.integer = i;
		sprintf(record.string, "RECORD-%d", i);
		fwrite(&record, sizeof(record), 1, fp);
	}
	fclose(fp);

	fp = fopen("records.dat", "r+");
	fseek(fp, 43*sizeof(record), SEEK_SET);
	fread(&record, sizeof(record), 1, fp);

	record.integer = 143;
	sprintf(record.string, "RECORD-%d", record.integer);

	fseek(fp, 43*sizeof(record), SEEK_SET);
	fwrite(&record, sizeof(record), 1, fp);
	fclose(fp);

	// file mmap to mem
	f = open("records.dat", O_RDWR);
	mapped = (RECORD *)mmap(0, NRECORDS*sizeof(record), PROT_READ|PROT_WRITE, MAP_SHARED, f, 0);
	
	getchar();
	// can upate change in other process
	mapped[43].integer = 243;
	sprintf(mapped[43].string, "RECORD-%d", mapped[43].integer);
	printf("update\n");

	getchar();
	msync((void*)mapped, NRECORDS*sizeof(record), MS_ASYNC);
	printf("write to file\n");

	getchar();
	munmap((void*)mapped, NRECORDS*sizeof(record));
	close(f);
	printf("close\n");
	exit(0);
}
